San Francisco Frontiers - Zverev scrapes through, Djokovic cruises to Shanghai Masters last 16

Novak Djokovic meanwhile powered through the third round, steamrolling Italian Flavio Cobolli 6-1, 6-2 in just over an hour.

Zverev's first set against 40th-ranked Griekspoor was overshadowed by the German's prolonged, expletive-laden exchange with the umpire when he disagreed with a call.

"I cannot be bothered to go to China in October, for you to screw up this... I cannot, this is not possible," Zverev ranted before tossing his racket across the court.

The world number three claimed the first set, but proceeded to fall apart in the second, with the Dutchman breaking him in the first, third and fifth game.

Zverev squandered an early lead in the third set tiebreak but an unforced error on Griekspoor's part saw him crowned winner.

"Every time I play him it's the most difficult match," said Zverev, saying there had been "not much difference" between them.

In contrast, Djokovic, a four-time Shanghai champion, completely dominated 30th-ranked Cobolli, breaking him in the second and sixth games of the first set.

"I can't complain about anything in the game, I was able to play aggressively," Djokovic said.

"I'm glad to be progressing through a tournament that has meant a lot to me in my career."

Fans waved Serbian flags and screamed in delight when the former world number one wrote part of his Chinese character name on a camera lens at the end of the match.

Djokovic will next play Russian Roman Safiullin, who upset Frances Tiafoe in three tight sets.

The end of that match also saw the American swear repeatedly at the umpire.

- Sinner-Shelton rematch -

The last 16 has finally fallen into place after days of schedule disruption due to rain.

Ben Shelton dispatched Spain's Roberto Carballes Baena 6-3, 6-4 to set up a Shanghai rematch with world number one Jannik Sinner of Italy.

The American knocked Sinner out of the tournament at the same stage last year.

"(I have) great memories from that match last year, I probably watched the tape of that a couple times so far this year... I'm really looking forward to it, that will be fun," Shelton said.

Sinner's chief rival Carlos Alcaraz, who beat him in the China Open final last week, will face veteran Gael Monfils.

The 38-year-old Frenchman defeated his younger and higher-ranked compatriot Ugo Humbert 7-6 (9/7), 2-6, 6-1.

Stefanos Tsitsipas recovered from a lapse in concentration in the second set to beat France's Alexandre Muller 6-3, 7-5.

The 26-year-old will meet world number five Daniil Medvedev in the next round.

"(Medvedev) likes courts like this. I feel like I'm getting better with time... What is missing is trying to get a good win under my belt," said the Greek, acknowledging his uneven recent form.

World number 10 Grigor Dimitrov is through after beating Australia's Alexei Popyrin 7-6 (7/5), 6-2.

The Bulgarian will play Czech teenager Jakub Mensik, who already ended one top 10 player's hopes when he sent Russia's Andrey Rublev home last week.

Zverev will face Belgium's David Goffin.

D.Brown--SFF